I think a lot depends on the group who have met on-line prior to the cruise, the length of the cruise (determines free time) etc. The only M&G's we've really been actively involved in were on two 12 day Mediterranean cruises. Those boards are really active with passengers getting together to share rides and excursions.
We met the first day at sail away and just gathered up around a pre-determined bar area and had drinks and finally got to put faces with names etc. The M&G lasted about an hour. We also had a second get together on the first sea day in one of the lounges on board. Our second group was really into this. Anyone who wanted, brought a small gift to exchange, usually something that represented their hometown or state; we had a poker game where everyone paid for a hand of cards and the best hands won the pot. And since this was a new ship with new cabin classes, we had a cabin crawl. Several passengers volunteered to host an open house at their cabin and we had a cabin from each class. We went around as a group and toured all the cabins, then went back for drinks and the final poker hand. It's a lot of fun, but takes someone who is willing to spend time organizing everything.
Been there - done that:
2005 Holiday, Sensation, Conquest,
2006 Conquest, Celebration, Holiday,
2007 Freedom Grand Med, Holiday
2008 Fantasy & Sensation,
2009 Fantasy, Holiday & Dream Grand Med
2010 Fantasy and B2B Elation
2011 Monarch of the Seas
2012 Booked - Breeze from Barcelona 12 days